Migration to Cities Returning to specifically demographic considerations, one that is of direct relevance to educational planners is internal migration. Although there are some instances of large net migrational gains by newly developing agricultural and mining areas, the overwhelming movement throughout the developing countries is from rural to urban areas. The significance of this movement for educational planning is that educational enrollment rates and the standards of educational services in most countries differ widely between rural and urban areas. It is not uncommon for educational enrollment rates to be as high as 80 percent in urban areas and as low as 10 percent in the more isolated rural areas. Educational planning that is predicated on a general steady rise in the average enrollment rate, with predictable trends in costs, may be rather drastically upset by high rates of migration into the cities. Children moving to the cities will need to be accommodated in schools at the 80 percent rate, not the 10 or 20 percent rate applying to the areas from which they came; after all, the greater educational opportunities in the cities may have been a major motive for the move. Costs in urban areas may also differ widely from those in rural areas, although it is hard to know on a priori grounds which would be the more expensive. Building costs may be substantially lower in rural areas if the villagers supply materials and voluntary labor; on the other hand, teachers may have to be given incentive payments to work there. In urban areas economies of scale may be realized when educational services are centralized in large schools; certainly, the standard of education provided should be superior to that in the small village schools. ISOLATING THE DEMOGRAPHIC OBSTACLE TO EDUCATIONAL GOALS Since the demographic obstacle is only one of a number of obstacles to attaining the ambitious educational goals the developing countries have set for themselves, it is necessary to attempt at least a rough measure of the importance of the demographic factor compared with others. There are many possible approaches, but in this section an attempt will be made to isolate the contribution of demographic trends to the rise in student numbers and teacher requirements, and in turn to rising costs, according to a variety of less and more ambitious educational targets. A very conservative target is merely to attempt to hold constant the proportion of children in school and to leave the pupil/teacher ratio unchanged. In this example, population growth is the only cause of rising student numbers, and of rising teacher requirements as well. It will result in increases of 50 percent or more in 15 years in any country whose youth population isican countries. Even when capital costs are included, teacher salaries constituted more than 60 percent of the total expenditures on education in all three continents (12, p. 35).